Materials Performance

AUG 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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14 AUGUST 2018 W W W.MATERIALSPERFORMANCE.COM
Chlorination Effect on Corrosion
of CuNi 90/10 in Seawater
C
u-based alloys are frequently
used for seawater piping
because of their resistance to
chloride pitting as well as their
antifouling characteristics. One Cu-based
alloy that is a preferred choice for its reli-
able performance in seawater applica-
tions (such as ballast water systems) is
CuNi 90/10 alloy (UNS C70600), which is
10 mass% of Ni alloyed to Cu.
Seawater for ballast water tanks is
normally treated with disinfectants (e.g.,
chlorine) by ballast water treatment sys-
tems to control invasive species in ballast
water. These invasive species can become
an environmental threat if released
untreated. A lthough the level of chlorine
is thought to inf luence corrosion of the
exposed metal, the criteria for chlorina-
tion dosing for seawater treatment have
yet to be well established for Cu-Ni pip-
ing, despite the long history of the alloy
being used for seawater systems.
When CuNi 90/10 alloy is used for sea-
water applications, it is ty pically recom-
mended that the seawater f low be main-
tained above a minimum f low rate of
1 m/s because slow f low makes it easier
for marine organisms to attach them-
selves to a metal surface, which can lead
to microbiologically inf luenced corrosion
(MIC). The f low rate of 1 m/s, however, is
rarely reached during the normal opera-
tion of certain seawater lines such as bal-
last water or f ire-f ighting systems, as
these systems f low only under specif ic
circumstances. Whether chlorinating
seawater is effective for suppressing
marine fouling and causes subsequent
corrosion of CuNi 90/10 alloy under stag-
nant conditions is unclear.
In CORROSION 2018 paper no. 11418,
"Effect of Chlorination on Corrosion of
90-10 Cu-Ni A lloy for Ballast Water Sys-
tem," authors Geunsu Jung, Byoung
Young Yoon, Jae Kwang Lee, and Chae-
Seon Lim with Samsung Heav y Industry
(Gyeongsangnam-do, South Korea) dis-
cuss a study where biofouling and corro-
sion issues for CuNi 90/10 alloy were
examined. Samples of the alloy were pre-
pared and immersed in bottled seawater
with various chlorination levels for up to
six months for two main research objec-
tives: to investigate the effective dosing
level of chlorination for CuNi 90/10 alloys
in seawater for effectively suppressing
marine fouling, and to evaluate the effect
of chlorination on the corrosion of CuNi
90/10 alloy in seawater under stagnant
conditions.
For the experiment, 10- by 30- by
5-mm samples were taken from a com-
mercially available CuNi 90/10 pipe and
prepared with various surface prepara-
tions: as received (as-is); mirror polished
to 1 μm using conventional methods (pol-
ished); and polished and immersed in
clean, fresh water for a month (condi-
tioned). The surface of the samples was
observed with an optical microscope
before the immersion test. The as-is and
conditioned samples showed similar sur-
face morphologies, although the condi-
tioned sample—through polishing—no
longer had grooves from the manufactur-
ing process, which could be seen on the
surface of the as-is sample. The glossy
surface of the polished sample was antici-
pated to be the most sensitive to corrosive
media as the protective oxide scales were
removed when the metal was polished.
The surface of the conditioned sample
MATERIAL MATTERS